Oil filtration involves the use of full flow filters that filter all oil on every pass through the filter and the use of by-pass oil filters in conjunction with the full flow filters. By-pass oil filters operate in parallel with the full flow filter and generally speaking have a greater filtration than the full flow filter.
Full flow elements filter all oil on every pass and work using an element operating like a series of small sieves, with tiny holes allowing oil to pass through, while filtering out particles too large to pass through the sieve-like element. However, the holes are not small enough to filter out many finer particles, and it is these particles that can cause significant engine wear over a period of time.
On the other hand a by-pass filter operates in parallel with the full flow filter and can be much finer to progressively remove finer contaminants not captured by the full flow filter. The use of a by-pass filter progressively filters all the oil, thus increasing the useful life of the oil and reducing engine wear.
Typical by-pass oil filters include pleated element by-pass elements. These filters employ a longitudinally pleated membrane that is prone to become clogged after just a few hours work. Many by-pass filters have a pressure release valve that opens when this occurs. Another type of by-pass filter is a multiple disc by-pass filter which lasts much longer than the pleated elements.
The most effective by-pass filters employ a roll of paper wound on a central core with the feedstock being forced edgewise through the roll of paper to progress longitudinally the full width of the paper media. The filtrate then flows to the central core where it exits the filter via a return pipe. The principle of operation of this type of filter is different to the sieve-like filter and the pleated filter in so far as the paper roll filter relies on oil passing between the layers of paper, instead of passing through a sieve-like element.
Early forms of paper roll filters employed a toilet roll fitted within a filter housing. Later more sophisticated forms employed a filter element comprising a disposable canister holding the paper roll.
Australian patent number 650176 describes a filter element of the cartridge type employing a filter housing with a filter element within the housing. The filter element employs a metal canister with a paper roll forced into the canister under pressure, the canister being open at one end, for entry of feedstock, with a base at the other end, the base having a central hole for the return pipe. A layer of gauze in the base of the canister allows the filtrate to flow across the base to the return pipe in the central core. The sides of the canister are ribbed to inhibit feed stock tracking down the sides of the canister. The canister also has three or four depressions circumferentially spaced depressions in the base to hold the gauze layer away from the base providing clear secondary flow passages below the gauze to improve flow of filtrate.
The paper is tightly wound providing a much finer effective filter than the pleated and disc type filters.
Nevertheless the metal canister has a number of disadvantages. The ribbed side wall is employed to inhibit tracking of feedstock down the side wall outside the filter media but limits the density of the media. While ribbing is effective to limit tracking the canister is prone to buckle under load while the paper is being pressed into the canister. This effectively limits the amount of sideways compression that can be applied to the paper roll to a maximum defined by the flex in the side walls of the can and the inherent flexing of the ribs as the paper is being pushed into position, the limit being canister failure.
The element construction of Australian patent number 650176 results in a five percent (5%) failure rate of canisters during insertion of the paper roll into the canisters. Also the canisters are prone to be crushed by oil pressure when in use particularly at cold start up when the oil is most viscous. Construction of canisters from thicker metal would make filters more expensive and the ribbing to prevent tracking more difficult to produce in a technical sense.
Since the metal canisters are disposable they must be designed for single use application at a reasonably economical cost. It would be desirable to provide an alternative that is competitively priced, improves filtration and is less likely to buckle under load.
As an alternative to metal canisters PCT/AU96/00762 describes a plastics filter canister suitable for repeated use to enable recycling of the canister. The canister is made sufficiently rigid such that the spent paper roll can be removed using a tool and a fresh roll inserted into the canister. Due to the construction of the metal canisters of patent number 650176 efforts to remove the spent rolls from the metal canisters would in most cases render the metal canisters unusable having been designed for use as a disposable element. The problem with the canister described in PCT/AU96/00762 is that the design is not amenable to mass production using injection moulding techniques. In addition, the base of the canister was designed with closely spaced concentric rings in an effort to reproduce the effect of the secondary flow through passages utilised in the base of the metal canister while supporting the media. While this arrangement was effective in supporting the filter media and gauze the flow characteristics were less than desirable.